Woops, I think I chose my words poorly. What I meant is that there is enough bandwidth on the fiber to saturate 20Mbit to everyone that the CO serves - not that the CO has enough bandwidth to provide that much throughput for everyone to the internet simultaneously. The fiber has a much higher capacity for carrying data than coax does because the way the signals are put on the medium are completely different.
With cable there's a really small amount of bandwidth (~200MHz, IIRC) that you have to share with everyone on your street. I think that the coax can only take a carrier of about 2GHz (which is where the ~200MHz of bandwidth comes from), and you have to be really careful about harmonic frequencies. If you are paying for 10MBit cable service, your modem can't even run that fast when the kids on your street get home from school - you're dead in the water before you even hit the bottleneck at the CO.
With a fiber, you can get a lot more bandwidth (not the frequency spread of the channel in this case, but data throughput) because you pick light of a specific frequency (the channel width is 0), and just slop a digital signal at whatever rate you like on top of it - as long as you don't get too close to the frequency of the light (factor of 10 is usually the closest you try to get - same as with analog), you'll get perfect reception. Even more cool is that you can combine frequencies which are really close together, because the channel width is infinitesimal.
Also, I'm paying about $3-4/megabit - no setup fees.